Electronic devices often contain components having voltage level requirements that are different from the voltage supplied by the electronic device's power supply. For example, the power supply in a modern personal computer typically provides a 12 volt direct current (12 VDC) output voltage but the computer's central processing unit (CPU) requires a much lower voltage, e.g., on the order of 1 VDC. To satisfy the lower voltage level requirement of the CPU, a direct-current to direct-current (DC-DC) converter is employed to step the 12 VDC down to the voltage level required of the CPU.
In many applications the DC-DC conversion is implemented using a switch-based DC-DC converter, like the switch-mode DC-DC converter 100 depicted in FIG. 1. The switch-mode DC-DC converter 100 comprises a switch 102, a diode 104, an inductor 106, and a capacitor 108. In stepping down the DC input voltage Vin to a lower DC output voltage Vout, the switch 102 (usually a power transistor) is controlled so that it opens and closes (i.e., turns OFF and ON) at a frequency fSW (typically in the range of 1 kHz to 1 MHz). In effect, the switching action “chops” the input voltage Vin into a voltage vS(t) having a rectangular waveform of commutation period T and duty cycle D.
For a given period T, the DC component of the chopped-up waveform vS(t) is equal to the average of the waveform vS(t) over that same period. In other words,
      D    ⁢                  ⁢    C    ⁢                  ⁢    Component    =                    1        T            ⁢                        ∫          0          T                ⁢                                            v              S                        ⁡                          (              t              )                                ⁢                      ⅆ            t                                =          DVin      .      The DC component is the desired DC output voltage Vout. However, the chopped-up waveform vS(t) also contains switching harmonics of the switching frequency fSW. The inductor 106 and capacitor 108 together comprise a low-pass filter that filters out the switching harmonics, so that only the desired DC output voltage Vout is passed to the output. The corner frequency of the low-pass filter is proportional to 1/√{square root over (LC)}. Accordingly, to be most effective at filtering out all switching harmonics and allow only the DC component to pass to the output, the corner frequency should be made as low as possible. This requires a high inductance inductor 106, a high capacitance capacitor 108, or both.
In some applications the load to which the switch-mode DC-DC converter 100 supplies power can change abruptly from an active state in which the load is performing a desired and useful function to an inactive state in which the load is not performing any useful function (or is only performing some lower rank function). One example of this is a computer configured as a server in a data center (i.e., “server farm”). In such an application the computer/server can draw 100 watts or more of power when active and computing but need only a fraction of that power (e.g., 100 or 1,000 times less) when inactive. To conserve energy, it would be desirable to supply power to the CPU only when the CPU is active and computing. Unfortunately, this is not possible when a conventional switch-mode DC-DC converter (like the conventional switch-mode DC-DC converter 100 in FIG. 1) is being used. The problem is that in data center applications CPUs transition between active and inactive states in very short times—on the order of nanoseconds. However, it can take milliseconds for the switch-mode DC-DC converter 100 to transition across the power supply range corresponding to the active and inactive states. Much of the required transition time is dedicated to charging and discharging the output low-pass filter (inductor 106/capacitor 108). The transition time could be shortened by reducing the values of the inductor 106 and capacitor 108. However, that would interfere with the ability to provide a flat and well-controlled DC output voltage Vout. Power to servers/computers in data centers is therefore usually always left on, even during periods of inactivity when the CPUs are not computing. This undesirably results in significant amounts of power being wasted during inactive periods. The amount of wasted power can be substantial, especially when all computers/servers in the data center are accounted for. Not only does the wasted power translate into higher energy costs for data center purveyors, it also contributes to excessive loading of the power grid, pollution, and harm to the environment.